Ultra Wide Band (UWB) radio may differ from conventional narrow band radio and spread-spectrum radio in that the bandwidth of the signal at −10 dB is typically greater than 20% of the center frequency, or at least 500 MHz wide. Further, in Ultra Wide Band Impulse Radio (UWB-IR), instead of transmitting a continuous carrier wave modulated with information or with the information combined with a spread code, which determines the bandwidth of the signal, a UWB radio transmits a series of very narrow impulses, typically less than ins in duration. These short time-domain impulses that are transformed into the frequency domain result in the ultra-wideband spectrum of UWB radio. Typically, the impulses are organized in time according to a certain time-hopping (TH) sequence (in general different for each user) for channelization and spectral smoothing purposes.
In the UWB-IR, the information conveyed on the signal may be coded through a transmitted reference (TR) modulation. TR modulation schemes were introduced to reduce the drawbacks of RAKE receivers, e.g. channel estimation or finding a suited template pulse waveform for correlation. In fact, for UWB-IR systems, the received signal energy is typically spread over a large number of multipaths and use a RAKE receiver with a high number of fingers, thereby becoming more complex to realize.
In TR modulation coding, two pulses are transmitted, the first pulse representing the reference pulse and the second pulse comprising the information. More precisely, DBPSK (Differential Binary Phase Shift Keying) modulation may be utilized to encode the information, meaning that, for instance, if a “0” is being transmitted, the polarity of the information pulse coincides with the polarity of the reference pulse, or vice versa if a “1” is being sent on the channel.
Transmitted reference Binary Pulse Position Modulation (TR-BPPM) coding is a specific example of a TR modulation scheme, in which two pulses are also transmitted. The first pulse represents the reference pulse, and the second pulse comprises the information. More precisely, with a BPPM (Binary Pulse Position Modulation) used to encode the information, for instance, if a “0” is being transmitted, the information pulse is delayed by a time amount D1 with respect to the reference pulse or, vice versa, if a “1” is being sent on the channel, the information pulse is delayed of a time amount D2 with respect to the reference pulse.
Assuming that the channel is stationary over a time frame comprising two pulses, the TR modulation scheme, and in particular, the TR-BPPM scheme, does not use explicit channel estimation since both the reference and information pulses undergo the same channel distortions. The TR modulated signal or the TR-BPPM signal then may be decoded by a differential-coherent receiver, which tends to behave as a quasi-ideal RAKE receiver, thus capturing a large portion of the energy associated with the multipath components.
One limitation of the TR modulation format is that it may not support non-coherent detection, since for the latter, the detector may not determine the polarity of the received pulses. One limitation of the TR-BPPM modulation format may be that it may not be demodulated by way of a single non-coherent detector, e.g. energy collector.